Soil Amendment

Shawnee Mission, KS

I'm starting a new shrub and flower bed in heavy clay soil. Plan to add bulk compost with a combination of tilling and double digging. What depth would provide a reasonable trade-off between labor involved and effectiveness for the amendment? Also, how much compost should be applied to get good soil composition?

Anne Arundel,, MD(Zone 7b)

Sorry you're not getting any responses.
A shovel will get you eight inches or so down without too much extra work. A lot of rooting will happen in eight inches. But then for individual shrubs you may want to amend each hole/ area. Though some recent sources say do not amend, force the new plant to get adjusted to the native soil. If you have the strength for double digging that sure would do a good job on it.

How much-- if you plan to mix it evenly in the top foot of soil, then one inch of compost will give you soil mix that is one-twelfth organic, about eight percent. I think I would do one to two inches,

Just two cents worth, hope you get some more opinions.

Bay City, MI(Zone 6a)

This is a copy/paste of something I wrote at another forum site about amending beneath a raised bed, but it applies to your situation as well:

I don't think you should disturb the underlying soil, unless you have a plan in place that allows you to control/drain the water that is sure to collect there. Here's why: If you amend the soil beneath your bed, water will flow not only downward through your bed, but also laterally into the bowl you've 'created' by amending the very heavy soil and making it more porous. IOW, you will have created the 'bathtub effect', trapping water that can only dissipate as quickly as the percolation rate of the surrounding soil allows.

This is going to occur anyway, naturally, and soon enough - even w/o you incorporating the OM. As soil life 'mixes' the OM from above into the clay, you're still going to gradually get this bathtub effect. No one can think that the soil 15-20-30 ft from your bed will become better-aerated at the same rate the soil beneath your bed and the added OM will, so even if you leave the bed progress w/o tilling the soil below, within a few years you will still get that bathtub effect. There is no sense in hastening the process by tilling.

However, and as mentioned, if you have a way to mechanically remove the water via a trench/ditch, tile, French drain, or a sump arrangement, it won't matter if you till/amend the soil below the bed now. Note through: water will actually dissipate faster from your (raised) beds if you don't amend. It will flow laterally across the top of the clay when the clay is saturated, and when the top surface is dry, it will percolate into the clay AND move laterally into the dry, upper soil surface faster than it will percolate into surrounding soils and out of the bathtub you've created by tilling.

You're caught in a catch 22 situation. While adding OM to the clay should improve tilth/friability, at the same time it creates a water retention basin unless you give it a way to escape.


Charlotte, NC(Zone 7b)

opgardner - I've not planted any shrubs lately, but I did put in a flower bed this past spring.

I have very heavy clay soil, and know what tapla means when he says that if you dig a hole in clay soil it will act like a bath tub that drains poorly.

I built a berm around the area and filled it with sheddred leaves. Then set each plant with its attached soil in the bed. Unfortunately the slugs had a "slug fest" and ate most of the plants, but some have survived.

My vegetables are all in raised 6" beds and in none of them have I disturbed the underlying clay soil. Their roots somehow manage to work themselves into the underlying clay

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